It is known that a foam is produced by extruding an expandable resin through a plurality of holes bored in a die to form expanded materials and fusing together the expanded materials while they are still soft. With this method only, however, it is not possible to produce a foam having a skin layer of high density. The surface of a foam not having such a skin layer of high density is easily scratched.
Hence, a method has been proposed to provide a skin layer of high density on the surface of a foam. The feature of this method resides in that a frame surrounding extrusion holes is fitted to the extrusion end surface of a die, and extruded materials are passed through the frame where they are expanded and fused together. If such a frame is provided and extruded materials are passed through the frame while passing a cooling medium through the interior of the frame, a foam which has a skin layer of high density on the entire surface thereof can be produced.
The above-proposed method is described in, for example, Japanese Patent Publication No. 35231/79. This method utilizes a frame in which the inner surfaces facing extruded materials are composed of planes extruding in parallel with each other in the extrusion direction. In more detail, the inner surfaces of the frame are formed by straight lines vertical to the extrusion end surface of a die. However, experiments have revealed that if a frame as described above is used, a skin layer of high density can be formed on the surface of a molded product, but when a number of extruded materials are fused together, big voids are formed in the interior of the molded product and, thus, it is difficult to produce a high quality foam. The reason is considered that when a number of extruded materials are fused together to form a molded product having a large cross-sectional area, gas resulting from a foaming agent is prevented from passing through the extruded materials and remains therebetween.
It has also been proposed to use a frame whose cross-sectional area is increased toward the outer end, in a method of molding a foam as described above. Japanese Patent Publication No. 33585/76, for example, discloses a frame having such inner surfaces whose cross-sectional area increases toward the outer end. In more detail, the cross-section of the frame is circular. A frame having such inner surfaces, however, fails to form a skin layer of high density over the entire surface of a foam. Hence, it has been desired to develop a convenient method to provide a high density skin layer uniformly over the entire surface of a foam.
Giving attention to the fact that conventional frames are such that the inner surfaces are extended in parallel with each other in the direction of extrusion, or the cross-sectional area is merely increased toward the outer end, studies have been made on the relation between the inclination of inner surfaces of a frame and the structure or properties of a foam as produced using the frame. As a result, it has been found that in order to provide a skin layer of high density on the surface of a foam while preventing the formation of voids in the interior of the foam, it is insufficient to merely incline the inner surfaces of the frame so that its cross-sectional area is increased toward the outer end, and it is necessary to design the frame so that the inclination angle of each inner surface to the extrusion direction is within the range of from 5.degree. to 30.degree.. It has further been confirmed that it is necessary to cool the inner surfaces and to bring extruded materials in contact with the thus-cooled inner surfaces.